Mia (Katie Jarvis), the 15-year-old heroine of writer-director Andrea Arnold's "Fish Tank," lives with her alcoholic mother and younger sister in a bleak housing project in Essex, east of London. She doesn't really attend school and has no close friends. Her only enjoyment, it seems, is dancing by herself. Wearing headphones and baggy sweats, she practices her hip hop moves in a boarded up apartment that looks out over the squalor she is attempting to dance away from.

Because Arnold hews the film so close to Mia's hurts, what might come across as a downer instead often has a startling immediacy. Only bad movies are depressing movies. "Fish Tank," for all its faults and vagaries, brings us up close to a fully realized human being, and that's revivifying.

Jarvis, who had no previous acting experience, was accidentally discovered by Arnold when she overhead the girl arguing with her boyfriend in a train station. Arnold was right to go with her instincts in casting an untrained unknown: Jarvis has an openness to the camera that a more accomplished performer might have lacked. Her acting has a moment-to-moment excitement because we can never tell what Mia will do.

Like most adolescents, only more so, Mia is a mass of contradictions. Her retort to a taunting girl from the projects is simple: She head butts her and breaks her nose. She also has a soft side. Eyeing a decrepit horse tethered to a vacant lot occupied by townies, she darts inside the property and attempts to set it free.

The symbolism of Mia unleashing a sick, corraled horse may be a bit thick, but it's not out of place here. Mia is penned in by the circumstances of her life. She revels in those moments when she breaks loose, not only in her dancing but also for the sheer love of speed when she suddenly sprints across the decaying landscape.

The sadness here is that, of course, Mia is mostly running away from things – from trouble. Her mother (Kierston Wareing), in her late 30s and already blowsy, cares so little for her that homelessness almost seems like a better option. When the mother brings home a handsome live-in boyfriend, Connor (Michael Fassbender), tensions tighten.

But something unexpected also happens. With his grinning easy-goingness, Connor soon becomes a father figure to the wary but needy Mia. (He tucks Mia into bed at night, innocently, and you get the impression this is the first time anybody ever did this for her.) Their evolving relationship is the most ambiguously powerful aspect of the movie. A more ham-fisted director would have villainized Connor for the ways he ultimately leads Mia on, but Arnold understands how people can be many things at once. The greyness that hangs over her neighborhood is symbolic, too: Nothing in this world is black and white.

The use of hand-held cameras to capture Mia's darting, frenetic temperament can be annoying. And Arnold overdoes the gloomy "rawness." She comes out of a neo-realist tradition of English filmmaking, best characterized by the films of Ken Loach, that equates grunge with truth. "Fish Tank" would be better if it wasn't so intent on serving up sordidness as social commentary.

But "Fish Tank," which won the 2009 Jury Prize in Cannes, has moments when Mia's tribulations really hit home. In one scene, she shows up for a dancing audition and soon realizes she is being judged for something sleazier. Mia was counting on this job to lift her out of her troubles, and the look on her face – the mix of bafflement and rage and resignation – is the emblem of her life and of this movie.